This invention relates to a foam glass having a crust layer on at least one side of a foam layer and is particularly concerned with improvements in the mechanical properties of the crust layer. The foam glass is of use mainly as an interior or exterior decorative material in buildings or even as an outer wall material and may be a reinforced foam glass using a metal netting or an alternative.
In the art of foam glass it is known to provide a crust layer on a major surface of a foam layer, i.e. foamed glass layer, with a view to protecting the foam layer and enhancing the tensile and flexural strength of the foam glass. For example, JP-A No. 50-123108 shows a foam glass having a vitreous crust layer of a very dense structure, which is formed by firing a mixture of glass particles and a fine powder of glass. JP-A No. 60-166239 shows a laminated foam glass having a foam layer whose porosity is 1-30 vol %. The crust layer is formed by firing a mixture of a major amount of a fine powder of glass and a minor amount of a coarsely granulated glass base material. This crust layer is considerably high in apparent specific gravity, viz. at least about 1.75 while the true specific gravity of glass is about 2.5.
The provision of a crust layer of a dense structure enhances rigidity of the foam glass but raises problems in different aspects. First, the dense crust layer lacks machinability so that cutting of the foam glass becomes almost impracticable unless the crust layer is omitted along planned cutting lines at the foam glass manufacturing stage. Even when cutting of the crusted foam glass is barely possible it is often that cracks propagate long from the cutting lines so that a considerable portion of the product becomes unacceptable. In another aspect, when the dense crust layer is externally and locally impacted resultant cracks are liable to propagate in many directions and sometimes cause collapse of the whole foam glass. On the other hand, if the crust layer is thoughtlessly made low in apparent specific gravity (hereinafter the term "bulk density" will be used in this sense) the surface hardness may not be sufficient so that the crust layer may be damaged relatively easily with, for example, a knife edge. Besides, formation of the crust layers of the prior art using a fine powder of glass is liable to suffer from locally insufficient fusion bonding of the crust layer to the foam layer and/or existence of unwanted voids in the crust layer. Such deficiencies will cause lowering of the strength of the foam glass as a whole.